1. Field of the Invention
This invention relates to a method of forming an elongated thin-walled cylindrical compact for a magnet used for forming a cylindrical compact from powdered magnet materials in order to manufacture a magnet having a thin-walled elongated cylindrical shape.
2. Description of the Prior Art
In recent years, super-quenched type magnets of RE (rare earth elements)-Fe system such as a magnet of Nd-Fe-B system have attracted special interest as a magnet having excellent magnetic properties.
Heretofore, the forming of a cylindrical compact for a magnet of this kind has been carried out by using a mold exemplified in FIGS. 5(a) and 5(b).
A mold 51 shown in FIGS. 5(a) and 5(b) is provided with a core pin 52 in the central position, a die 54 disposed around the core pin 52 through a molding cavity 53, a pressing punch 55 disposed on one side of the molding cavity 53 and a receiving punch 56 disposed on another side of the molding cavity 53, which also functions as a knock out punch.
In case of forming the cylindrical compact by using the mold 51 having the aforementioned structure, first of all, powdered magnet material is obtained by crushing properly thin plate or flake-shaped base alloy which is turned out by super-quenching a molton alloy having predetermined chemical compositions such as a magnet of Nb-Fe-B system (may be added with Co, Ga, Dy and so on according to demand) through a molton metal quenching process.
Next, the aforementioned powdered magnet material or a green compact 57 is charged into the molding cavity 53 formed by the core pin 52, the die 54 and the receiving punch 56 as shown in FIG. 5(a). The green compact is a preparatory compact formed by cold-pressing the powdered magnet material so that its theoretical density ratio may be within a range of not more than 90%, generally within a range of 70% to 80% or so. The powdered magnet material or the green compact 57 is pressed and compressed by the pressing punch 55 with pressing speed Vp as shown in FIG. 5(b) at a temperature between 600.degree. C. and 900.degree. C., more preferably between 700.degree. C. and 800.degree. C. in a vacuum or in an atmosphere of an inert gas such as argon, and the cylindrical compact 58 is obtained by compressing until the density reaches a theoritical density ratio of not less than 98%, more preferably 99%.
A cylindrical shaped permanent magnet is obtained by magnetizing the aforementioned cylindrical compact 58 in a magnetic field.
However, there is a problem since it is difficult to form a thin-walled elongated cylindrical compact for obtaining a thin-walled and long-sized cylindrical magnet by the conventional forming method of the cylindrical compact.
Namely, the cylindrical magnet of this kind is utilized mostly in small-sized electric motors of automatizing appartus used in factories, offices and so on, and has been expected to make the motors lighter and smaller by using the thin-walled elongated cylindrical magnets making the most of their excellent magnetic properties. However, it is difficult to form the cylindrical compact into an elongated thin-walled shape present conditions in spite of the fact that it is necessary to obtain the thin-walled elongated cylindrical magnet. Therefore, the thin-walled and long-sized cylindrical compact is obtained by grinding the compact formed into an elongated thick-walled cylindrical shape or by joining some compacts formed into short, thin-walled cylindrical shape in the axial direction, for example.
Further explanation will be given on basis of FIG. 6 showing the molding cavity 53 of the mold 51 shown in FIG. 5 after enlargement.
In FIG. 6, at the time of pressing the powdered magnet material or the green compact of the powdered magnet material 57 after charging it into the molding cavity 53 formed by the core pin 52, the die 54 and the receiving punch 56, if the pressing speed of the pressing punch 55 is represented by Vp, pressing speed V in the downward direction for the powdered material at the position away upwardly from the upper face 56a of the receiving punch 56 is given by: EQU V=Vp(x/1)
wherein l is height of the cylindrical compact. The pressure corresponding to frictional forces .mu.Pc between the compact and the core pin 52 and .mu.Pd between the compact and the die 54 (.mu. is coefficient of friction) in the upward direction is required for this part of the compact in addition to the pressure required for the compressive deformation of the compact in itself.
Accordingly, pressure P required to form the compact including above is given by following equation of theoretical calculation: EQU P=(2.155e.sup.2 .mu.x/t -1.155)k
wherein .mu. is coefficient of friction between the mold and the compact (approximately 0.08), e is the base of natural logarithm and k is the pressure required to deform the compact in itself. It is necessary to apply a pressure higher than this in order to form the compact in practice.
On the one side, the pressure k required for compressive deformation of the powdered magnet material of Nd-Fe-B system is approximately 8 kgf/mm.sup.2 and the calculated results of the pressure P required to form the compact in this case are shown in FIG. 3. In this case, x/t is read along the transverse axis with a scale on the lower side in FIG. 3.
As it is clear from FIG. 3, it is necessary to apply the high pressure as much as 76 kgf/mm.sup.2 to the pressurized face 55a of the pressing punch 55 in order to obtain the compact having a height being ten times its wall thickness (l/t=10), for example.
On the other side, is desirable to carry out the forming at a temperature between 600.degree. C. and 900.degree. C., so that the mold 51 to be used is also preheated at the temperature. In practice it is impossible to form the thin-walled elongated cylindrical compact because an elongated pressing punch 55 is capable of resisting a pressure no more than 25 kgf/mm.sup.2 without buckling in the case of making the pressing punch 55 with heat resisting alloy, and merely possible to resist the pressure of about 40 kgf/mm.sup.2 at the aforementioned temperature even when the pressing punch 55 is made with hard metal. Accordingly, the forming limit of the cylindrical compact is 4.3 to 6.5 according to the quality of the pressing punch 55 indicating by the ratio of the length to the wall thickness (l/t) of the cylindrical compact.